The present invention is directed toward braid-reinforced composites, resin compositions and processes for their preparation.
Braid-reinforced composites are useful in a variety of applications, particularly those benefiting from materials that are not only strong but also relatively lightweight. Despite the processing challenges presented thereby, braid is often the reinforcement of choice in composite materials that serve a wide variety of market applications.
For example, braid is used in aerospace applications like aircraft engine containment, aircraft propeller blades, missile nose cones and bodies, self-lubricating bearings, control surfaces, aircraft engine stator vanes, aircraft ducting and tubing and satellite components. Braid is typically the primary, load bearing reinforcement in automobile cross beams, automobile air bags and restraint devices, commercial furniture, industrial rollers, lamp and utility poles. Braid is often a partial reinforcement in structures like shipping containers and boat hulls. Braid is being used extensively for prosthetic limbs and orthotic braces, surgical devices like endoscopes and catheters and implantable devices such as splints and stents. Recreational equipment utilizing braided reinforcement includes wind surfing masts, snow boards, water skis, snow skis, wake boards, sail masts, boat hulls, hockey sticks, golf shafts, bicycle components, baseball bats, tennis and other racquets, and kayak and canoe paddles.
In addition to the benefits associated with its relative strength and weight, braid is commonly used in composites simply because it enables cost-efficient composite processing in certain applications. Braided reinforcements present composite fabricators with a variety of opportunities to be more cost effective because of their unique combination of attributes. For example, because of its “finger trap” configuration, biaxial braid easily and repeatably expands open to fit over molding tools or cores, accommodating straight, uniform cross-section forms as well as non-linear, irregular cross section components. Braid is, in essence, a near net-shape preform. Just as socks rolled up in a drawer do not look like feet, braids shipped on reels or festooned in cartons do not look like net shape preforms. However, just like pulling socks on your feet, braids slip onto tools and cores with speed, ease, and a high degree of repeatability.
At a fundamental level, a braid is a system of three or more fibers intertwined in such a way that no two fibers are twisted around one another. In practical terms, “braid” refers to a family of fibers continuously woven on the bias. Currently, there are four main braid forms and four main braid architectures available. The acute angle measured from the axis of the braid to the axis of the bias fibers is called the “braid angle.” This angle is also referred to as the “fiber angle” or the “bias angle.” The braiding process allows for the introduction of axial fibers between the woven bias fibers. These axial fibers are generally not crimped by the weaving process.
Braided fiber architectures resemble a hybrid of filament winding and weaving. Like filament winding, a tubular braid features seamless fiber continuity from end to end of a part. Like woven materials, braided fibers are mechanically interlocked with one another. The combination, however, is quite extraordinary. When functioning as a composite reinforcement, braid is capable of exhibiting remarkable properties because it is highly efficient in distributing loads. Because all the fibers within a braided structure are continuous and mechanically locked, braid has a natural mechanism that evenly distributes load throughout the structure.
Braid's efficient distribution of loads also makes braided structures very impact resistant. Since all the fibers in the structure are involved in a loading event, braid has the capacity to absorb a great deal of energy in the event of a failure. This is why braid is used as fan blade containment in commercial aircraft and in energy absorbing crash structures in formula one racing cars.
Braided structures are also excellent with respect to fatigue. Like a filament wound structure, braided fibers are coiled into a helix just like wire in a spring. The difference, however, is the mechanical interlocking. As a structure is exposed to high fatigue cycles, cracks will propagate through the matrix of filament wound structures. While micro-cracking can occur in a braided structure, the propagation is typically arrested at the intersections of the reinforcing fibers. This is why braid is the reinforcement of choice for aircraft propellers and stator vanes in jet engines.
Braid greatly improves interlaminar shear properties when nested together with other braids. While interlaminar adhesion is typically no different from other reinforcement products, the layers move together. As a result, it is very rare for cracks to form and propagate between layers of braided reinforcement. Since braids are woven on the bias, they provide very efficient reinforcement for parts that are subjected to torsional loads. As such, braid is an ideal reinforcement for drive shafts and other torque transfer components such as flanged hubs.
Despite all of their potential and capabilities, braid-reinforced composites present unique processing challenges. In preparing braid-reinforced composites, a braid form must be impregnated with a suitable resin. Without adequate and uniform impregnation, properties of the resulting braid-reinforced composites are compromised. There exists a continued need for improving the process and materials used for formation of braid-reinforced composites in order to maximum the potential benefits thereof.
Essential to formation of a braid-reinforced composite is inclusion of at least one braid. Braid is most commonly manufactured and used as a freestanding form with a constant braid angle for a given diameter. In this form, it is supplied on reels or festooned in cartons. In composite manufacturing, tubular braids are typically expanded open diametrically, applied to a molding tool or core, snugged down using the “finger trap” effect, impregnated with resin and then consolidated. During pultrusion, tubular braids are continuously fed over a die mandrel to produce hollow cross sections impregnated with resin. Flat braids are used primarily for selective reinforcement, such as tabbing in boat building and strengthening specific areas in pultruded profiles.
Adequate and uniform impregnation of braid forms is challenging, but important. For example, voids in the fully cured product (which can be introduced, for example, due to volatilization of solvents or inadequate impregnation of resin into the braid) are often areas of stress concentration that can contribute to premature failure of the reinforced composite when under an applied stress.
In an effort to overcome the difficulties in obtaining adequate infiltration of the resin through the complex arrangement of fibers within a braid, fiber tows have first been coated with the resin and then braided. However, this method has not proven sufficient to efficiently and adequately braid fibers coated with a resin when the resin has too much tack or inflexibility. Further, this method requires that the user have access to not only resources for impregnating the fibers forming the braid, but also the resources for arranging the fibers into a braid form. Thus, obviously formulation and processing latitude is constricted when using this method.
Some of the above-noted disadvantages are overcome by impregnating resin onto pre-braided fiber forms. According to these efforts, an initial resin is coated onto the braided form. Often, the initial resin is a B-stage thermosetting resin that forms a prepreg once combined with the braided form. As is well known in the art of prepreg manufacture, B-stage refers to an intermediate stage in the reaction of certain thermosetting resins, in which the resin swells when in contact with certain liquids and softens when heated, but may not entirely dissolve or fuse. B-stage thermosetting resins are generally needed to facilitate transportation and maintenance of such a prepreg structure until final cure of the thermosetting resin in conventional fiber-reinforced composites. After configuration of such a prepreg into its intended structure, the thermoset resin of the prepreg is fully cured to a state having essentially irreversible hardening and insolubility.
Nevertheless, efforts to impregnate resin into pre-braided fiber forms have also not proven sufficient to efficiently and adequately achieve products having the intermediate and final properties desired. Although mixtures of thermosetting resin and its associated curative can be heated to a temperature sufficient to decrease their viscosity and improve fiber impregnation, when the curative is heat-activated this process may result in premature cure of the thermosetting composition if not handled appropriately. The type of curative used may also make it impractical to heat the composition to the necessary temperature for imparting the desired viscosity.
Also, while lower viscosity compositions facilitate more rapid and adequate impregnation of pre-braided fiber forms, there is a competing need for the resin coating impregnated on the braid to remain substantially and uniformly impregnated throughout the braid until final curing. To facilitate this need, upon impregnating the resin composition on the fibers, it is typically at least partially cured (e.g., to a B-stage). However, B-stage resins are generally less flexible than resins that have not been at least partially cured; and, as noted above, one of the advantages of a braid is its sock-like flexibility. Therefore, it is desirable to retain that flexibility even when the braid is impregnated with a resin during preparation of braid-reinforced composites. Without that functionality, it is more difficult to process intermediate braid-reinforced resin articles into final braid-reinforced composites.
Conventional prepregs are often made using solvent-based methods. According to solvent-based methods, viscosity of the resin system is decreased by the addition of sufficient amounts of solvent to enable adequate impregnation of the fibers. While this method facilitates fiber impregnation without the use of heat, the solvent is typically removed subsequently via heating. Generally, the use of solvents makes the process more expensive, less environmentally friendly, and potentially hazardous. Furthermore, the use of a solvent-based method can result in the undesirable retention of solvent within the final prepreg product. During subsequent cure of the prepreg product, residual solvent is prone to volatilization (e.g., upon heating). Volatilization of the solvent can create unwanted voids within the final composite laminate.
Hot-melt processing is an alternative method that has been used when forming conventional prepregs. Not surprisingly, methods of this type are often preferred as compared to solvent-based methods. However, because conventional prepregs are often based on resins having a relatively high viscosity, heat in excess of 60° C. is often needed to sufficiently reduce the viscosity so that the resin can adequately infiltrate the fibers during formation of the prepreg using hot-melt processing techniques. Yet, as discussed above, when the resin's curative is heat-activated this process may result in premature cure of the thermosetting composition if not handled appropriately. Thus, the type of curative used may make it impractical to heat the composition to the necessary temperature for imparting the desired viscosity.
While known efforts, such as those described above, may result in fiber-reinforced composites useful for certain applications, adequate braid-reinforced composites are not known to be readily available and useful. Not surprisingly, those skilled in the art are challenged to produce braid-reinforced composites efficiently and in a way that maximizes the benefits obtainable by use of complex fiber forms such as braids.